This invention relates to a signal processing system and particularly to a method and a circuit suitable for use in a flowmeter to process composite fluid flow signals which are sensed in a pipeline conduit by a flow sensor having a nonlinear frequency response. The invention is specifically useful with nonlinear thermal sensors without frequency compensation and for producing signals corresponding to the fluid flow rate through the conduit.
Prior art fluid dynamic flowmeters often use thermal sensors, such as heated thermistors, hot wires, hot film and semiconductor materials, to sense perturbations in a fluid flow about a bluff body in a pipeline. Ordinarily, the sensor is electrically heated to a temperature elevated above that of the fluid in the pipe. Flow signals are then sensed by the cooling and reheating of the sensor under control fluid velocity variations or vortex shedding phenomena about the bluff body in the pipe.
A recognized deficiency of such thermal sensors is that they have relatively poor thermal frequency response and, as a consequence, the signal amplitudes which they produce at higher frequencies are less than those produced at lower frequencies. The nonlinear amplitude-frequency response of thermal type sensors has long been known and illustratively is treated in "Thermistors, Their Theory, Manufacture and Application" published Jan. 1960 by the Institution of Electrical Engineers, R. W. A. Scarr and R. A. Setterington and the articles cited in the bibliography of that publication.
In view of the nonlinear characteristics of thermal sensors, the prior art has utilized circuitry for linearizing or compensating for the disparities in the thermal response over operating frequency ranges of interest. Such circuitry is disclosed, for example, in R. F. Mahon-C. L. McMurtrie U.S. Pat. No. 3,535,927 issued Oct. 27, 1970. It functions to equalize the amplitude of undesired low frequency fluid flow rate noise signals with desired higher frequency fluid flow rate signals both of which are sensed by the thermal sensor.
A problem in such linearizing or compensating arrangements is that the frequency response characteristic of the thermal sensor is a strong function of the fluid media in which it is immersed. Thus, to achieve good linearization or compensation the sensing arrangement would need to be tested in the fluid in which it would operate and the compensation or linearization tailored for that arrangement. If an appropriate linearization is used to cover most liquid media, then the quality of the signal generated by such an arrangement may be sufficiently degraded as to limit the dynamic flow range that could be handled without errors.
It is therefore apparent that a need exists for a technical advance which aids in reducing the necessity of linearization or frequency compensating arrangements for thermal sensor in flowmeter equipment and provides alternatives to thermal frequency compensation techniques.